1. Field of the Invention
The present invention relates to an imaging apparatus such as a car-mounted camera, and more particularly to an imaging apparatus using the NTSC system.
2. Description of the Background Art
To accommodate display devices using the NTSC system, car-mounted cameras typically take images at 30 fps with an imaging element such as a CMOS sensor. However, when a car-mounted camera takes an image of a display device such as an on-street LED traffic signal, the following problem occurs.
The LED traffic signal is driven by a drive voltage coming from a full-wave rectified commercial AC source. Therefore, the signal blinks on and off in an extremely short time cycle that is not visually recognizable. Thus, in a frame, when a charge accumulation time of the imaging element and a light-out period of the signal overlap, the signal appears to be out in the frame even though a red, blue or green light is actually on. When multiple such frames continue, dozens of image frames in each of which a signal in a light-out state is imaged are produced. This is a particularly serious problem for a drive recorder. The drive recorder records images taken by the car-mounted camera, and is widely used to analyze accidents. The true state of the signal when an accident occurs is important in determining liability for the accident.
In order to solve the problem, Japanese published unexamined application No. 2009-181339 (JP-2009-181339-A) discloses a method of offsetting a frame frequency of the imaging element by a predetermined frequency relative to a frame frequency of the NTSC system to set a frame frequency (offset frame frequency). When an LED signal is imaged, dozens of image frames in each of which a signal in a light-up or a light-out state is imaged are not produced. However, the frame frequency of an image signal is out of the frame frequency of the NTSC system, and cannot be displayed in a display device of the NTSC system.
In order to solve this problem, it is conceivable that image data taken from the imaging element be stored in a frame memory at a frame frequency (offset frame frequency) taken out of the frame frequency of the NTSC system, and read out from the frame memory at the frame frequency of the NTSC system to be produced in the display device. However, the following fresh problem then occurs.
The image data is typically read out from the imaging element by a progressive method, whereas the display device of the NTSC system uses an interlace method. Therefore, a time for writing one frame of image data in the frame memory and a time for reading out one frame of image data from the frame memory differ from each other, and a comb noise specific to the interlace method is made even when the frame memory is a double buffer. The comb noise is a pectinate image shift generated in a frame image formed of an odd-number field and an even-number field of a different frame.
For these reasons, a need exists for an imaging apparatus less susceptible to a blink cycle of lighting devices such as LED traffic signals, is capable of using display devices using the NTSC system, and can prevent occurrence of comb noise.